Current emission control regulations necessitate the use of catalysts in the exhaust systems of automotive vehicles in order to convert carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) produced during engine operation into harmless exhaust gasses. Vehicles equipped with diesel or lean gasoline engines offer the benefits of increased fuel economy. Such vehicles have to be equipped with lean exhaust aftertreatment devices such as, for example, Active Lean NOx Catalysts (ALNC), which are capable of continuously reducing NOx emissions, even in an oxygen rich environment. In order to maximize NOx reduction in the ALNC, a hydrocarbon-based reductant, such as fuel (HC), has to be added to the exhaust gas entering the device. However, introducing fuel as a reductant reduces overall vehicle fuel economy. Therefore, in order to achieve high levels of NOx conversion in the ALNC while concurrently minimizing the fuel penalty, it is important to optimize usage of injected reductant. In this regard, it is known that improved NOx conversion can be achieved by introducing the reductant in vapor rather than liquid form due to better distribution and mixing of the reductant with the exhaust gas entering the NOx reduction device.
One such system is described in U.S. Pat. No. 5,771,689, wherein a reductant is introduced into the exhaust gas via an evaporator device that has a hollow body with a heating element protruding into its interior. The evaporator device protrudes into the wall of the exhaust pipe upstream of the catalyst. The reductant is introduced so that it flows through the narrow space between the hollow body and the heating element until it reaches the tip of the heating element from where it enters the exhaust pipe in vapor form and mixes with the exhaust gas entering the catalyst.
The inventors herein have recognized several disadvantages with this approach. Namely, if delivery of the reductant has been shut off, or reduced, as dictated by the operating conditions, some reductant may remain in the annular space, in contact with the heating element, and may therefore clog up the opening around the heating device by carbonation of the residual fuel. Such carbon build up may lead to a blockage of the passage at the tip by which the vaporized fuel enters the exhaust stream. Further, there is a delay in introducing the reductant into the exhaust gas stream due to the time it takes for the reductant to travel down the length of the heating element. Additionally, durability of the heating element is reduced because its temperature is not controlled and adjusted based on operating conditions, and due to soot contamination. Yet another disadvantage of the prior art approach is that extra power is consumed due to the above-mentioned lack of temperature control.